The cognitive function of the brain depends critically on the synaptic connections formed between neurons and their target cells. Inappropriate formation and development of synaptic connections can cause neuronal dysfunction and neurological disorders. Hence, understanding synaptic development and function remains a major goal for neuroscientists. As part of our long-term research interests, we aim to study the in vivo role of epsin and its signaling pathways in synaptic development at the neuromuscular junction (NMJ) of the fruit fly, Drosophila melanogaster. Our studies show that the Drosophila epsin (called liquid facets or lqf) gene plays a novel role in synaptic growth at NMJ. lqf mutations alter bouton shape, increase bouton size, and reduce synaptic transmission. Furthermore, we show that Lqf acts as a specific substrate of the deubiquitinating enzyme Faf to promote synaptic growth. Finally, we show that the level of MAP1B as well tubulin is reduced on microtubules (MTs) in a considerable number of synaptic boutons in the lqf mutant. In contrast, overexpression of Lqf in neurons appears to induce MT bundling in synaptic boutons. Through genetic screens, we have isolated a number of lqf modifiers, including the MAP1B mutant futsch and the lissencephaly 1 (dlis1) mutant affecting the MT cytoskeleton. These observations prompted us to hypothesize that Lqf regulates synaptic growth and function in an MT- and ubiquitin-dependent fashion. To test this hypothesis and to gain further insights into Lqf and Lis1 function, we propose three specific aims. In the 1st Aim, we will examine how lqf mutations and overexpression of Lqf affect the MT cytoskeleton using immuocytochemistry, transmission electron microscopy, and live imaging. We will also determine the structural domain that mediates Lqf function at synapses. In the 2nd Aim, we will determine the synaptic role of Lis1 and the genetic and biochemical relationship between Lqf and Lis1. Finally, we propose to characterize one of the novel lqf modifiers to further dissect the genetic pathway mediating the synaptic function of the Lqf-Lis1 complex. Lqf (epsin) and its partners (Futsch and DLis1) are highly conserved between flies and mammals. Further, the synaptic roles for epsin and DLis1 remain poorly understood. Thus, information derived from the proposed studies is expected to have broad biological and clinical significance.